1. Field of the Invention
The present invention relates generally to the field of security seal monitoring. More particularly, the present invention relates to time-domain reflectometry monitoring of security seals. Specifically, a preferred implementation of the present invention includes the use of fiber optic loops that are equipped with independent transmitters/receivers so as to permit the near real-time monitoring of a large number of security seals distributed among a plurality of such loops with a single time-domain reflectometer, thereby obviating the need for visual inspection of the seals. The present invention thus relates to a seal monitoring system of the type that can be termed active.
2. Discussion of the Related Art
Historically, the Department of Energy (DOE) has been responsible for the long-term storage and protection of large quantities of Special Nuclear Material (SNM). SNM is stored in individual containers located in vaults. Security measures and inventory cycles are the means by which DOE ensures that SNM remains within the containers where it is stored and verifies that it is not disturbed in any manner.
One method for monitoring the storage of SNM within the containers is to affix tamper-indicating devices (TID's) to the container closures. These TID's are typically mechanical devices such as wire cables that are looped through the closures in such a way that they are destroyed in a most visible manner when the closures are opened. Although these TID's do not provide physical security per se, they are an indispensable aid to material accountability and inventory.
To verify the integrity of a TID, it is usually necessary to have persons visually inspect and physically test the TID. In this regard the TID can be considered a passive seal because the TID itself cannot alert an alarm system when it has been breached. The discovery of a breached seal does not necessarily reveal when the breach occurred, since the breach could have occurred anytime between inspections. Furthermore, because passive seals must be visually inspected, verification of the seals is a labor-intensive endeavor that often requires exposure of personnel to radiation hazards. Therefore, what is needed is a system that actively monitors seals.
One unsatisfactory previously recognized approach, in an attempt to solve the above-discussed problems involves incorporating all the seals in a given area of a storage facility into an active monitoring circuit. However, a disadvantage of this previously recognized approach is that when such a circuit is broken, it is not possible to know exactly which container in the alarmed area has been opened. Therefore, what is also needed is a solution that can actively indicate exactly which seal(s) in an area have been breached.
To address the above-discussed disadvantage, another unsatisfactory previously recognized approach involves fitting each of the seals in a storage facility with a separate active circuit. However, a disadvantage of this previously recognized approach is complexity, especially where there are large number of seals to be monitored. Further, this previously recognized approach is costly. Therefore, what is also needed is a solution that meets the above-discussed requirements in a simple and more cost effective manner.
The typical use of an optical time-domain reflectometer (OTDR) is to determine the location of a discontinuity or a large transmission loss, such as that imposed by a faulty component, in an optical fiber. An OTDR can discriminate against small losses such as those caused by satisfactory components. An OTDR can typically find faults in optical fibers at fiber distances of many kilometers from the OTDR. The detection distance is limited primarily by the attenuation loss intrinsic to the fiber, the number of components inserted in the fiber, and the dynamic range of the OTDR. Similarly, the spatial resolution of an OTDR can be as coarse as several meters or as precise as a fraction of a centimeter. The spatial resolution is limited primarily by the duration of the optical pulse generated by the OTDR and the temporal resolution of the photodetector and its associated electronics.
An OTDR provides an excellent means of establishing a system of active seals. Basically, if an optical fiber connector is present at an arbitrary point along an optical fiber as a seal or tamper indicating device, then the OTDR can determine whether the seal (connector) is secure (closed) or breached (open). The OTDR can uniquely identify any one of tens, hundreds or thousands of such seals along an optical fiber because the temporal position of the Fresnel reflection from the seal (connector) and present on the waveform generated by the OTDR electronics has a one-to-one correspondence with the spatial position of the seal along the optical fiber. This system functions as an active seal system because the OTDR can remotely, immediately and automatically detect that a seal has been breached.
An improvement on the idea of a series of seals on a single optical fiber cable is to introduce a 1-by-n switch after the OTDR. This switch provides a means for implementing multiple optical fiber cables, each with series of seals. In the vocabulary of OTDR technology, a connector or other entity that causes a Fresnel reflection or other transmission loss is referred to as a "feature." A series of optical fiber cables that are joined by connectors (and that therefore contain features) are referred to as a "link." A system of links that can be accessed by the OTDR by a switch or other means of multiplexing is referred to as a "system."
The OTDR system described above suffers from two significant limitations. First, the disconnection of a seal in link disables the ability of the OTDR to monitor seals beyond the disconnected seal since the light pulse is terminated by the open seal. Second, in a multiplexed system the OTDR can monitor only one link at a time, thereby eliminating the ability of the OTDR to immediately detect any breached seal in a non-monitored link.
The below-referenced U.S. Patents disclose embodiments that were satisfactory for the purposes for which they were intended. U.S. Pat. No. 4,095,872, to Stieff et al., entitled "Security sealing system using fiber optics," was issued Jun. 20, 1978. U.S. Pat. No. 4,106,849 to Stieff, entitled "Fiber optic seal," was issued Aug. 15, 1978. U.S. Pat. No. 4,130,341 to Stieff, entitled "Fiber optic seal apparatus," was issued Dec. 19, 1978. U.S. Pat. No 4,161,348 to Ulrich, entitled "Preassembled fiber optic security seal," was issued Jul. 17, 1979. U.S. Pat. No. 4,297,684 to Butter, entitled "Fiber optic intruder alarm system," was issued Oct. 27, 1981. U.S. Pat. No. 4,367,460 to Hodara, entitled "Intrusion sensor using optic fiber," was issued Jan. 4, 1983. U.S. Pat. No. 4,447,123 to Page et al., entitled "Fiber optic security system including a fiber optic seal and an electronic verifier," was issued May 8, 1984. U.S. Pat. No. 4,729,626 to Stieff, entitled "Self-locking fiber optic seal," was issued Mar. 8, 1988. The entire contents of all the above-referenced U.S. Patents are hereby expressly incorporated by reference into the present application.
Within this application several publications are referenced by superscripts composed of Arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the present invention and illustrating the state of the art.